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Background: GPS First put into practical use in the 90’s. More commonly used in the 21st century GPS is for navigation, syncing computer networks time, missile guidance Some applications that make use of GPS are Garmin Car Navigation Systems, Google maps, mobile apps GPS satellites are maintained by the Air force and can be used by anybody

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Global Positioning System (GPS): How it works At least 24 operational GPS satellites in orbit 12 hour orbit 11,000 miles above earth Atomic clock Most accurate time and frequency standards known Synchronized, send signals at same time

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Global Positioning System (GPS): How it works cont’d. Satellites send data to earth which are picked up by a receiver Signals arrive at different times based on the distance from the satellite L1 ( MHz) Receiver needs to determine distance to four satellites Determines 3-dimensional position Does not send out a signal But how does the receiver determine its distance from each satellite?

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Global Positioning System (GPS): How it works cont’d. To calculate distance: Distance = Speed Time Speed ≈ Speed of Light How to determine time? Receiver ’ s clock becomes synchronized to Coordinated Universal Time (UTC) by tracking four or more satellites Each satellite transmits a unique “ pseudo random ” code at extremely precise time intervals Receiver knows each satellite ’ s pseudo random code and when they are sent Receiver determines the time delay it takes to match the expected satellite pseudo random code with the received pseudo random code Time Delay = Time!

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Global Positioning System (GPS): Sources of Error Atmospheric Error Speed of light is only a constant in a vacuum Charged Particles in the Ionosphere Water Molecules in the Troposphere Ephemeris Error Error that effects the satellite’s orbit (ephemeris) Caused by the gravitational pull of the sun, moon, and the pressure caused by solar radiation Error monitored by the Department of Defense (DoD) and broadcasted to the GPS satellites Multipath Error Timing error from signals bouncing off of objects such as buildings or mountains Can be reduced by signal rejection techniques How can we reduce errors caused by the atmosphere?

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Global Positioning System (GPS): Error Correction: DGPS DGPS = Differential GPS Basic Idea: Use known locations as reference locations Exact Position is known, compare to the location determined by GPS Develop error correction data by using the difference of the exact location and the GPS determined location Broadcast error correction data to local GPS receivers (receivers within 200km of the reference station) Error correction can remove errors caused by the atmosphere—makes GPS data more accurate!

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Global Positioning System (GPS): Error Correction: WAAS Wide Area Augmentation System (WAAS) WAAS is an example of DGPS Also referred to as a Satellite Based Augmentation System (SBAS) Developed by the Federal Aviation Administration (FAA) Uses a network of ground based stations in North America and Hawaii Measures variations in satellite signals Relays error to geostationary WAAS satellites Used to improve accuracy and integrity of data Independent systems being developed in Europe (Galileo), Asia, and India.

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Global Positioning System (GPS): NMEA Cont’d. Requirements Contain complete position, velocity, and time (PVT) data Independent of other messages Begin with a ‘$’, end with a ‘\n’ Content separated by commas No longer than 80 characters

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Inertial Navigation System The use of inertial measurements in navigation Measurements come from inertial sensors such as: Accelerometers Gyroscopes Very accurate over short term Errors integrate with time

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Mechanical Accelerometers Mass suspended in a case by a pair of springs Mass suspended in a case by a pair of springs Acceleration along the axis of the springs displaces the mass. Acceleration along the axis of the springs displaces the mass. This displacement is proportional to the applied acceleration This displacement is proportional to the applied acceleration Picture from “Basic Inertial Navigation” by Sherryl Stoval

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Capacitive Accelerometers Sense a change in capacitance with respect to acceleration Sense a change in capacitance with respect to acceleration Diaphragm acts as a mass that undergoes flexure Diaphragm acts as a mass that undergoes flexure Two fixed plates sandwich diaphragm, creating two capacitors Two fixed plates sandwich diaphragm, creating two capacitors Change in capacitance Change in capacitance by altering distance between by altering distance between two plates Most common form Most common form e011.html

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Navigation Equations The navigation equations can be represented as (Shin, 2001):

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Navigation Equations Body  NED

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Navigation Equations GPS and INS need to be in the same reference frame for proper measurements. GPS data is in Earth Centered Earth Fixed (ECEF) INS data is in Body frame and has to be translated to the North-East-Down frame Body  NED, ECEF  NED Picture from “Accuracy and Improvement of Low Cost INS/GPS for Land Applications” by Shin

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Integration of GPS and INS Different integration levels: Loosely Coupled Corrects errors in the IMU and INS Does not correct GPS Tightly Coupled Corrects both INS and GPS errors Kalman filtering integrates both systems to achieve a more accurate overall system